Technology at the molecular scale is built on two complementary strategies for manipulating matter and energy. Static nonstructures, exemplified by carbon nanotubes,1-3 metallofullerenes,4-8 dendrimers,9-14 and supramolecular assemblies,15-19 are designed molecular systems that exhibit specific useful properties, including electrical or thermal conductivity, tensile strength, and photochromism. Dynamic nanomachines,20-25 on the other hand, are molecular systems designed to perform some type of work when powered by an energy source, such as an added chemical reagent, electron transfer induced by an electrical potential, or absorption of light. Both types of molecular-scale devices are needed for the development of a comprehensive nanotechnology.21 The construction of dynamic molecular machines is the next challenge for chemists, physicists, and materials scientists.26 
One approach to developing nanoscale systems that perform work reproducibly has been to study and exploit natural biological machines, such as the proteins myosin27-31 and kinesin.27,32-35 Biomolecular motors are now emerging as candidates for power sources integrated into artificial nanomechanical structures. As an example, Montemagno and coworkers,36-39 have demonstrated this approach using an F1-ATPase rotary motor driving a >>1000 nm long nickel rod “propeller” powered by ATP chemical fuel.
Harnessing biomolecular motors offers the advantages of proven operation and a phenomenal energy efficiency that has been optimized in naturally occurring biological systems.39 However, these motors have a number of characteristics that limit their utility for powering artificial nanostructures in many applications. Among these are a lack of precise temporal control for energizing/deenergizing the motor, a complex and degradable fuel source, and relatively low speed operation (˜8 Hz for the rotary ATPase motor). Most restrictive is the narrow range of environmental conditions (e.g. solvent, pH, temperature) which biomolecular motors can tolerate. Biomolecular motors have dimensions of several tens of nm, making them awkward to incorporate into structures where the motor footprint or actuator travel is desired on the scale of individual small molecules (˜1 nm).
Synthetic biomimetic molecular motors potentially other a much smaller size, robustness, and the possibility of tailoring the design features for targeted performance criteria. There has not yet been a demonstration of a practical synthetic nanoscale motor that can deliver power to a nanoscale mechanical load, but a number of approaches are being explored. The construction of interlocking molecular systems (“molecular shuttles”) in which a cyclic molecule can be induced to move along a chain of atoms in response to physical or chemical stimuli others the possibility of controllable linear motion.22,23,25,34,40-44 Several groups recently have reported the successful design of molecules that exhibit unidirectional rotary motion, essentially prototypes of molecular motors that are biomimetic analogs of rotary biological motors in, for example, flagelia. Kelly and co-workers45-48 have developed a system comprised of a helicene\ratchet” attached to a triptycene\rotor” whose rotary motion in one direction is driven by a sequence of chemical reactions. The research groups of Feringa and Harada49-52 have introduced a system in which unidirectional rotary motion is achieved by a four-step process involving two thermal chemical reactions and two photochemical reactions. Michl has conceptualized a large-scale molecular “propeller” that should exhibit unidirectional motion under bombardment from a beam of He atoms according to computer simulations.53,54 The structures envisioned by these groups demonstrate the feasibility of achieving unidirectional rotational motion despite concerns that such motion may violate the second law of thermodynamics.55 In fact, the success of these systems is based on creating a non-equilibrium system whose relaxation to equilibrium is responsible for the observed dynamics.
Both of the existing synthetic motor prototypes depend in part or in whole on thermal chemical reactions to achieve rotary motion.48,50 These designs typically have low repetition rates (on the order of seconds), require thermal cycling, and potentially poor temporal and positional control. A more desirable motor prototype would have high repetition rates (> kHz), and would be ideally controllable with a long operational lifetime. Many of these design goals can be attained using either an electron-transfer event or the absorption of light as a power source.25 